In recent years, organic EL devices have been noted as a candidate for flat panel display having high brightness not found so far and research and development therefor have been made vigorously. The organic EL device has the structure in which a light emitting layer containing a light emitting material is put between two electrodes and holes injected from an anode and electrons injected from a cathode are re-combined to emit light in the light emitting layer.
The organic EL device includes two types. One uses a charge transporting material with addition of a fluorescent dye reported by Tang (C. W. Tang), et al. (Journal of the Applied Physics (J. Appl. Phys.), 65, 3610 (1989)) and the other uses the fluorescent dye itself as the fluorescent layer (for example, device disclosed in Japanese Journal of the Applied Physics (Jpn. J. Appl. Phys.), 27, L269 (1988)).
The organic EL device using the fluorescent dye itself as the light emitting layer is generally classified into further three types. In the first type, the light emitting layer is put between a hole transporting layer and an electron transporting layer to form three layers. In the second type, a hole transporting layer and a light emitting layer are laminated into two layers. In the third type, an electron transporting layer and a light emitting layer are laminated into two layers. It has been known that the luminous efficiency of the organic EL device is improved by laminating in two layers or three layers as described above.
The electron transporting layer in the organic EL device of each of the constitutions described above contains an electron conducting compound as one of charge transporting materials and has a function of conducting electrons injected from a cathode to a light emitting layer. While the hole injecting layer and the hole transporting layer is a layer containing a hole conducting compound as one of charge transporting materials and have a function of conducting the holes injected from the anode to the light emitting layer, when the hole transporting layer and/or hole injecting layer is interposed between the anode and the light emitting layer, more holes are injected at a lower electric field into the light emitting layer and, in addition, electrons injected from the cathode or the electron injecting layer can be confined in the light emitting layer as well, so that an organic EL device of excellent luminescent performance such as improved luminous efficiency can be obtained.
As the hole transporting material and the hole injecting material as one of charge transporting materials used for the hole transporting layer and the hole injecting layer of the organic EL device described above while various materials have been known including triphenylamine derivatives, few of the materials are suitable to practical use at present. For example, while N,N′-diphenyl-N,N′-di(3-methylphenyl)-4,4′-diaminobiphenyl (hereinafter simply referred to as TPD) has been reported (Applied Physics Letter, Vol. 57, No. 6, pp 531, 1990), the compound is poor in the heat stability and involves a problem in view of the life of the device or the like. Various triphenylamines are described also in U.S. Pat. Nos. 5,047,687, 4,047,948 and 4,536,457, JP-B-6-32307, JP-A-5-234681, JP-A-5-239455, JP-A-8-87122 and JP-A-8-259940. Among them, N,N′-di(1-naphthyl)-N,N′-diphenyl-4,4′-diaminobiphenyl (hereinafter simply referred to as NPD) has been known as a compound obtained by improving heat stability of TPD.
JP-A-4-308688, JP-A-6-1972 and Advanced Material, vol. 6, p 677, 1994 describe star-burst amine derivatives, and JP-A-7-126226, JP-A-7-126615, JP-A-7-331238, JP-A-7-97355, JP-A-8-48656 and JP-A-8-100172 and the Journal of the Chemical Society, Chemical Communication p 2175, 1996 and the like also describe hole transporting materials for hole injecting materials improved with heat resistance. Among them, 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine as a kind of star burst amine derivatives (hereinafter simply referred to as MTDATA) has been used generally as the hole injecting material for organic EL devices.
However, organic EL devices using the triarylamine derivatives described above have no performance corresponding to the recent demand for higher performance at present. One of the major reasons is insufficient ability of hole injection and, particularly, large ionization potential value of the hole transporting material. For efficiently injecting holes into the organic layer of the organic EL device, it is important for a relation between the work function of the electrode and the ionization potential of the organic compound and lower ionization potential value is more preferred. For this purpose, it has been demanded for the development of hole transporting material having small ionization potential value for improving the performance of the organic EL device.
As described above, since the existent charge transporting materials used for organic EL devices have no practically sufficient performance or involve a problem not capable of coping with higher efficiency, it has been demanded for improving the efficiency and the life of the organic EL device by the use of excellent materials.
On the other hand, Heterocyclic Communications, vol. 2, pp 117, 1996 shows a diazapentacene derivative as the electron donating compound (donor compound) but it contains no descriptions regarding the organic EL device.